The invention relates to a method and arrangement for forming a transmission signal. The invention is advantageously applied in mobile stations, where a good linearity and low power consumption of an RF transmitter are important features.
In general, mobile station systems apply QAM (Quadrature Amplitude Modulation), where the information is modulated with the transmission signal phase and amplitude. There are also known systems where two different modulation methods are implemented. For instance the NADC (North American Digital Cellular) system applies analog NBFM (Narrow Band Frequency Modulation) and digital, combined amplitude and phase modulation OQPSK (Offset Quadrature Phase Shift Keying). In these modulation arrangements it is important that both the amplitude and phase of the signal to be transmitted are transferred through the transmission chain in a linear fashion.
In order to achieve good linearity, the transmitters generally use power amplifiers of classes A and AB. The transistors of these amplifiers are kept continuously in the linear range by means of biasing. A drawback in these arrangements, in particular with reference to mobile stations, is the high power consumption of the amplifiers, i.e. poor efficiency, which is caused by high idle current consumption. Low power consumption of the amplifier is an important feature in mobile stations, because said devices are usually operated with small-size batteries.
In order to cut power consumption, it is possible to use amplifiers of classes B, C, D, E and F. In amplifiers of these classes, the transistors in idle state are nearly or completely without bias voltage, wherefore their idle current consumption is very low. However, a drawback with the amplifiers of said classes is their nonlinearity.
In the prior art there is known, from the U.S. Pat. Nos. 5,144,258 and 5,105,164 a power amplifier arrangement whereby the linearity of a nonlinear amplifier is improved by comparing the amplitudes of incoming and outgoing signals, and the gain of a nonlinear amplifier is controlled on the basis of said comparison. Thus it is attempted to control the amplifier output in proportion to the control voltage amplitude by means of feedback. The drawback of this solution, however, is that although the absolute value of the output amplitude would be made linear in relation to the absolute value of the control signal amplitude, the output signal phase still behaves in a nonlinear fashion with respect to the control signal phase.
From the patent publication U.S. Pat. No. 5,740,521, there is known an arrangement for forming transmission signal, wherein by means of a phase-locked loop, it is attempted to correct the error occurring in the frequency of an intermediate-frequency oscillator. However, the suggested procedure does not correct the phase error possibly caused by a nonlinear amplifier.
The object of the invention is to eliminate the above mentioned drawbacks by introducing a novel arrangement, where the nonlinearity of the power amplifier is corrected by means of both phase and amplitude feedback. Now the employed amplifier can be a nonlinear amplifier with a high efficiency, and yet a good phase and amplitude linearity is achieved by means of the arrangement.
One idea of the invention is that the modulation signal is formed of separate baseband signals controlling the phase and amplitude modulation. A phase control signal controls a phase-locked loop, wherefrom there is further obtained a transmission frequency signal to the nonlinear RF amplifier. An amplitude control signal controls the amplification of the RF amplifier. The signal obtained from the RF amplifier output is used as the feedback signal, for separately correcting both the amplitude control and the phase control, so that the transfer function of the arrangement is made linear as regards both amplitude and phase.
In the specification of the invention and in the appended claims, xe2x80x9cphase modulationxe2x80x9d may also mean frequency modulation, unless informed to the contrary.
The method according to the invention for forming an amplitude and phase modulated transmission signal, wherein
there are created a baseband amplitude modulation signal and a phase modulation signal,
there is formed a carrier frequency signal,
the phase of said carrier frequency signal is controlled on the basis of said phase modulation signal,
the carrier frequency signal is amplified in order to form a transmission signal, and
the amplification of said carrier frequency signal is controlled on the basis of said amplitude modulation signal, is characterised in that in addition to the above,
the phase of the carrier frequency signal also is controlled on the basis of the carrier frequency signal received after the last amplification step in order to correct the transmission signal phase, and
the absolute value of said amplification is controlled on the basis of the carrier frequency signal received after the last amplification step in order to correct the amplitude of the transmission signal.
The arrangement according to the invention for forming an amplitude and phase modulated transmission signal, comprising
means for forming a baseband amplitude modulation signal and a phase modulation signal,
an oscillator for creating a carrier frequency signal,
means for controlling the phase of the oscillator output signal on the basis of the phase modulation signal,
an amplifier for amplifying the carrier frequency signal and for forming a transmission signal, as well as
means for controlling the amplification of said amplifier on the basis of said amplitude modulation signal, is characterised in that the arrangement also comprises
means for controlling the phase of the output signal of said oscillator on the basis of the output signal of substantially the last amplification step of said amplifier in order to correct the transmission signal phase, and
means for controlling the amplification of said amplifier on the basis of the output signal of substantially the last amplification step in order to correct the transmission signal amplitude.
Preferred embodiments of the invention are presented in the dependent claims.